How is allowing developers to use alternate payment platforms "bad for customers"? Or, if not on the phone, at least to allow them to add funds to their account from the browser/another platform and use it in their iOS app?
Surely we can just apply the Windows argument of "well they should get antivirus software and stop browsing shady sites" and go about our day having improved the landscape for the tech-literate, while having increased the risk other people take on by using their phone if they don't know the first thing about ensuring the apps they download are safe or privacy-preserving. That's why people buy iPhone - you literally do not have to worry about malicious App Store apps.
Because those "alternate payment platforms" don't prioritise consumer protection.
Deliveroo did an update their software that wiped out my login settings. They didn't support apple login, so I lost access to my account. They won't recover my email because of a special character in it, and so they continued to charge me for their "plus" service every month, and avoided any emails I tried to send asking to stop (asking me to "log in" to change my payment settings!).
Requiring apps use Apple's channel would have protected me from that experience.